1. Field of the Disclosure
The disclosure relates to a multiuser multiple input multiple output (MU MIMO) system. Particularly, the disclosure relates to a channel information feedback method and an apparatus thereof used for beamforming and power allocation of a MU MIMO system.
2. Description of Related Art
With the development of wireless communication technology, multiple input multiple output (MIMO) systems are widely used to increase transmission capacity and transmission quality of communication systems. The MIMO system can support multiple users to achieve a multiuser MIMO (MU MIMO) system. However, in order to achieve better spatial multiplexing with a high transmission rate, a transmitting end (for example, a base station) has to perform optimal beamforming and power allocation according to channel state. Therefore, a receiving end (for example, a user end) has to measure channel state information (CSI) and feed back the CSI to the transmitting end.
The CSI of the MIMO system can be represented by a matrix having a plurality of complex elements (which is referred to as a CSI matrix hereinafter). The CSI matrix contains a large amount of information, and a magnitude of the information amount thereof relates to the number of antennas (including transmitting antennas and receiving antennas) and the number of users. Therefore, regarding a finite bit rate uplink channel, if the amount of information of the CSI matrix is too large, the uplink channel can be fully occupied.
Presently, some wireless communication standards adopt a codebook-based pre-coding method, by which the receiving end selects a mostly suitable pre-coding matrix from a codebook according to the measured CSI matrix, and feeds back an index thereof to the transmitting end, the transmitting end obtains the pre-coding matrix by looking up the codebook according to the index and pre-codes data to be transmitted, and then transmits data to the receiving end. The pre-coding method only uses a finite bit feedback index; however, by using the predetermined codebook, an optimal transmission performed in an environment with feeding back real-time channel state cannot be achieved, so that system performance is relatively poor.
Generally, the receiving end directly feeds back the CSI matrix, and the transmitting end calculates an optimal beamforming matrix according to the CSI matrix, so as to transmit data to the receiving end to achieve a better performance. A commonly used beamforming technique is a zero-forcing (ZF) beamforming technique, the ZF beamforming technique is used to prevent signals transmitted to a user end by the transmitting end from being influenced by signals transmitted to the other users. Such ZF beamforming technique only requires a direction part of the CSI matrix (which is referred to as a subspace matrix). The transmitting end receives the subspace matrix and performs optimal beamforming on the data to be transmitted, and accordingly transmits data to the receiving end.
The ZF beamforming technique does not require a magnitude part (which is referred to as a magnitude matrix) of the CSI matrix, so that the receiving end is only required to feed back the subspace matrix of the CSI matrix to the transmitting end. However, there are limitations on the number of antennas of the ZF beamforming technique, and when channels corresponding to two users are the same, the transmitting end can only transmit data to one of the users.
Moreover, non-ZF beamforming techniques or other types of the beamforming and power allocation techniques are also provided, and performance of some non-ZF beamforming technique is better than that of the ZF beamforming technique. However, these techniques all require the complete CSI matrix (including the direction part and the magnitude part), so that in case of the finite bit rate of the uplink channel, the receiving end generally uses vector quantization to quantize the whole CSI matrix, and feed back the quantized CSI matrix to the transmitting end.